March
14, 2005 ó Since late February the mid-Atlantic and Northeast have been
plagued with wind chills that have sent many back in the house for their
hat and gloves. On the minds of many in the East, "Where is it
all coming from?" Scientists at the NOAA
Climate Prediction Center have isolated the negative phase of the
North Atlantic Oscillation, or NAO,
as a major contributing factor to this late-season winter weather. (Click
NOAA satellite image for larger view of latest negative phase of the
North Atlantic Oscillation, or NAO, as of March 14,2005. Click
here to view latest image. Please credit “NOAA.”)

The North
Atlantic jet stream patterns have a major influence on winter weather
across the eastern United States. These jet stream patterns affect the
frequency, severity and duration of cold-air outbreaks from Canada.
They also help to determine when and where winter storms will form and
what regions will be affected.

"Winter
weather patterns are quite variable in part because the North Atlantic
jet stream pattern can take on a variety of configurations from one
day to the next, and from one week to the next," said Gerry Bell,
meteorologist at the NOAA Climate Prediction Center. Scientists have
long known there are certain preferred configurations of the jet stream
over the North Atlantic Ocean, with the two dominant ones referred to
as the "negative" and "positive" phase of the NAO.

New
York’s Central Park Sets Snowfall Record

Snowfall
this season in New York's Central Park reached 40 inches following
the 1.5 inches of snow that fell on Friday and Saturday. This
marks the first time New York has recorded three consecutive snow
seasons with 40 inches or more since records began in 1869. During
the 2003/2004 season, 42.6 inches fell following the 2002/2003
season with 49.3 inches. An average season has 22.4 inches.(Previous story)

"The
so-called negative phase of the NAO has prevailed for the past three
weeks," said Bell. In this phase, the North Atlantic jet stream
is shifted south of normal over the eastern U.S., and the upstream polar
jet stream coming southward from Canada is stronger than normal. These
conditions favor more frequent and more intense cold-air outbreaks across
the eastern U.S. Jet streams also determine where winter storms will
form, and how intense they will be. A negative phase of the NAO and
a southward shift of the North Atlantic jet stream sometimes means more
frequent winter storms for the eastern United States, as has been observed
in past weeks.

In contrast,
the positive phase of the NAO features a northward shift of the North
Atlantic jet stream pattern over the eastern U.S., and a reduced flow
of cold air from Canada. In this phase, milder air overspreads much
of the eastern U.S., and the potential for Nor'easters is reduced.

The jet
stream patterns captured by the two phases of the NAO can be very persistent,
sometimes dominating for 10-20 years. For example, the negative phase
of the NAO controlled the circulation during the 1950s-1960s, and led
to colder and more severe winters during that period. In contrast, the
milder-than-normal winters between 1980 and 2000 were associated with
frequent positive phases of the NAO.

"Week-to-week
and even day-to-day variations in the NAO can have a major influence
on winter storms along the East Coast," said Louis W. Uccellini,
director of the NOAA National Centers for Environmental Prediction and
the NOAA National Weather Service's winter weather expert. "The
negative NAO is now a climate signal that many forecasters are looking
for in their day-to-day predictions of winter weather along the East
Coast. The negative NAO is a key factor in sustaining cold air in the
northeast U.S., which allows precipitation to reach the ground as snow
rather than rain, when a major storm is affecting the East Coast,"
he added.

The NOAA
Climate Prediction Center and Hydrometeorological Prediction Center
routinely monitor and predict the phases of the NAO out to one and two
weeks. Bell notes, "It is currently not possible to predict changes
in the NAO beyond two weeks. This adds to the challenge in predicting
winter season scenarios for the East Coast."

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